Spark-type and compression-type internal combustion engines can generate several forms of nitrogen oxide (NOx) gasses as natural byproducts of the fuel combustion process. NOx gasses may be present in an engine exhaust stream in various forms, including nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O). To reduce the levels of the various NOx gasses present in vehicle tailpipe emissions, modern vehicles are equipped with various catalytic devices that reduce the NOx gasses to inert compounds.
Within a vehicle exhaust system, a suitable reductant such as ammonia or urea may be added at precisely controlled rates to the NOx-containing exhaust stream, and then passed over a catalytic device. Catalytic action subsequently converts the NOx gasses into nitrogen and water. Sensors positioned within the exhaust stream measure NOx levels to determine the conversion efficiency of any catalytic device used for this purpose. However, conventional NOx conversion efficiency monitoring algorithms may be less than optimally robust under certain operating and maintenance conditions.